The present invention relations to a circuit for controlling low frequency noise currents drawn from DC power sources by loads requiring substantial AC energy at low frequencies.
In telecommunication systems, fans are generally used to reduce equipment's temperature rise. The fan's speed is controlled by a motor, which is usually controlled by a voltage applied across its terminal. A motor speed controller is used to control the motor speed and maintain a desired voltage level. The motor speed controller is often required because the telecommunication system's battery voltage often varies over a wide range. For example, the voltage range of a 48V system can vary from 36V to 75V, with a transient up to 100V. Thus, the motor speed controller provides a constant voltage to the motor terminals to maintain the constant motor speed when the battery voltage changes over a wide range.
It is well known that a direct current (DC) motor will draw a current that has a significant low frequency alternating current (AC) component. For example, referring to FIG. 1, a waveform is resulting from a DC motor with 24V/1.5 A (36 W) output is illustrated. With 24V DC voltage applied to the motor, the DC value of the motor current is about 1.5 A. However, the AC component is 2.5 A peak to peak. The frequency of the AC current is about 275 Hz. Additionally, there are significant harmonic components in the waveform.
The motor speed controller is a DC to DC switching power supply with an inherently fast response for maintaining the output voltage at a constant level. The low frequency AC component of the motor current is reflected in the input of the motor speed controller. Therefore, the input current of the motor speed controller has a similar shape, which includes a significant low frequency AC component.
Existing telecommunication equipment standards require that the input current of the motor controller must meet both wide band and voice band noise limits, which are much lower than the AC current, described above. In the example illustrated in FIG. 1, the noise of the AC current is approximately 43 dbrnc and the standard requires that the noise should be less than 9 dbrnc. For a 3 db noise margin, a 37 dbrnc (43−9+3) attenuation of the AC current is required.
Referring to FIG. 2, a standard method for attenuating the AC current is illustrated. An inductor-capacitor (LC) low frequency filter is added between the telecommunications equipment power source and the motor speed controller. A current Imotor between the motor speed controller and the fan comprises a large low frequency AC current component. There is also a significant amount of low frequency AC current in an input current IQ of the motor speed controller. The value of capacitor C1 and inductor L1 is very large, thus the impedance of capacitor C1, XC1, is very small and the impedance of L1, XL1, is very large. Therefore, a low frequency AC current Iac flows through the capacitor C1 and a DC current Idc flows through the inductor L1. As a result, the input battery current, Ibat is DC.
However, since the frequency of the AC current is very low, approximately 275 Hz in the above example, the values of L1 and C1 should be very large. In the above example, in order to meet the requirements set by the standard, the values for L1 and C1 are selected as C1=2200 uF and L1=1000 uH. The size of such an inductor is fairly large, increasing its cost and space requirement. Furthermore, although the normal operating voltage for the system is 48V, the voltage rating for the capacitor C1 should be higher than 48V because the input voltage may range between 36V and 75V or even wider. Such a capacitor further adds to the cost and space requirement for the system.
Referring to FIG. 3, an alternate solution to the problem is illustrated. In this solution, an active filter is placed between the telecommunications equipment power source and the motor speed controller. An active filter controller controls the voltage across MOSFET Q in such a way that the terminal characteristics of the MOSFET, that is the voltage across drain to source and the current through the drain to source, behaves like a large inductor.
However, the problem with the active filter is a large power loss in the MOSFET Q. The voltage across the MOSFET Q (VDS) should be higher than the worst case ripple voltage across capacitor C1 in order for the active filter to operate properly. The worst case ripple voltage happens at minimum input voltage when the input current is at maximum value. Another limitation is the gain of the MOSFET Q is very small when it operates at low drain to source voltage region. In a practical situation, the voltage across MOSFET Q should be larger than 1V, and is typically between 2–3 V. This causes significant power loss in the MOSFET Q.
Further, it should be noted that in other applications, it is required to attenuate low frequency noise generated by electronic loads other than a motor of a fan. Such loads present the same difficulties as that described for fan.
It is an object of the present invention to obviate or mitigate at least some of the above mentioned disadvantages.